1. Field of the Invention
The present invention relates to an image conversion apparatus for converting an input image having a process color component and a spot color component, which is represented by density data for colors, into a halftone dot image represented by halftone dot data for colors of a process color, and an image conversion program storage medium storing an image conversion program, when executed in a computer, which causes the computer to operate as the image conversion apparatus as mentioned above.
2. Description of the Related Art
Hitherto, when an image printing is performed using a printing machine, there is performed such a matter that prior to performing a printing, a printer and the like is used to create a proof image similar to a printed image obtained through printing of the printing machine, and it is examined whether a desired printed matter can be obtained. The printer for creating such a proof image and a computer system into which such a printer is incorporated are referred to as a proofer.
As a result of the examination of the created proof image, when an operator is aware of any unfavorable point, the regulation of conditions having any effect on editing and another final printed matter is carried out to correct unfavorable points, and the proof image is created over again.
In this manner, repetition of the creation of the proof image and the alteration of the conditions makes it possible to finally determine a printing condition for obtaining a desired printed matter.
Here, when a printing machine is used to perform an image printing, process color inks of three colors of C (cyan), M (magenta) and Y (yellow), or four colors of K (black) in addition to C, M and Y, are used to represent various colors by halftone dots. Alternatively, it happens that ink having a color intended to be expressed per se, which is referred to as the spot color ink, is used in addition to those process color inks.
When a proof image similar to the color of a printed image using the spot color ink is created, usually, a printer does not use such a spot color ink, but uses only process color ink of for instance C, M, Y and K to express a color.
In this case, since the printed image is expressed by the process color ink and the spot color ink, a proof image is created in accordance with density data represented by process colors only obtained in such a manner that the color of the spot color ink is separated into process colors of C, M, Y and K, and density data originally represented by process colors of C, M, Y and K is combined (usually added) with density data represented by C, M, Y and K when the spot color is separated into the process colors of C, M, Y and K, for each process color.
Of the proofers for creating a proof image, there is a proofer for creating the proof image in accordance with halftone dot data. In the event that a proof image for a printed image including the spot color is created using such a proofer, the density data represented by process colors only obtained in the manner as mentioned above is converted into the halftone dot data for each process color, so that the proof image is created in accordance with the halftone dot data.
When a proof image of a printed image including the spot color is obtained, it is possible to create the proof image, as mentioned above, in such a manner that the spot color is separated into process colors, and the process colors thus obtained are combined with the components of the original process colors for each color, so that the proof image is created in accordance with image data of the combined process color. However, according to the proof image thus obtained, it is quite unclear as to whether an image of a certain portion of the printed image is represented by components of the spot color or the process color. This brings about a problem that a proof image, which omits important information for evaluation of finish of the printed image, is inadvertently obtained.
In order to solve this problem, it is considered to provide a proofer in which not only the process color but also the spot color are used to create a proof image. However, preparation of such a proofer involves great increment of the cost. Further, in view of the fact that there are many sorts in the spot color, and thus it is difficult to prepare a number of spot colors which is the same as the spot colors for printing. Accordingly, it is unrealistic to provide such a proofer.